The present invention concerns electrolytic compositions based on polymers, for electrochemical generators. More specifically, the invention is directed to aprotic electrolytic compositions characterized in that they consist of at least one alkali metal salt and a polymer matrix consisting of a polyether and at least another polymer matrix, which are separated microscopically, and are swollen by means of at least one polar aprotic organic solvent, said solvent or mixture of solvents being unequally distributed between the matrices.
During the last ten years, lithium batteries of the primary and rechargeable type have been the object of a considerable number of research and development works. The intent was to develop a battery which is safe, inexpensive, having a large energetic content and good electrochemical performances. In this context, a plurality of a battery designs were developed to meet different applications, such as micro-electronics, telecommunications, portable computers and electrical vehicles, to name only a few.
Electrochemical batteries or generators, whether rechargeable or not, are all made of an anode which can consist of a metal such as lithium, or an insertion compound which is reversible towards lithium, such as carbon, a cathode which consists of an insertion compound which is reversible towards lithium such as cobalt oxide, a mechanical separator placed in between the electrodes and an electrolytic component. The term electrolytic component means any material placed inside the generator and which is used as ionic transport except electrode materials in which the ions Li+ may be displaced at the level of the separator as well as in at least one composite electrode. During the discharge or charge of the generator, the electrolytic component ensures the transport of ionic species through the entire generator from one electrode to the other and even inside the composite electrodes. In lithium batteries, the electrolytic component is generally in the form of a liquid which is called liquid electrolyte or a dry or gel polymer matrix which may also act as mechanical separator.
When the electrolytic component is in liquid form, it consists of all alkali metal salt which is dissolved in an aprotic solvent. In the case of a lithium generator, the more common salts are LiPF6, LiBF4 and LiN(SO2CF3)2 and the polar aprotic solvents may be selected from propylene carbonate, ethylene carbonate, xcex3-butyrolactone and 1,3-dioxolane or their analogs to name only a few. At the level of the separator, the liquid electrolyte is generally impregnated in a porous polymer matrix which is inert towards the aprotic solvent used, or in a fiberglass paper. The use of a liquid electrolyte which is impregnated in an inert polymer matrix enables to preserve a sufficient ionic mobility to reach a level of conductivity of the order of 10xe2x88x923 Scmxe2x88x921 at 25xc2x0 C. At the level of the composite electrodes, when the latter are made of an insertion material which is bound by a polymer matrix which is towards aprotic solvents, which have only little interaction with the latter, the liquid electrolyte compensates for the porosity of the electrode. Examples of batteries utilizing a liquid electrolytic component are found U.S. Pat. No. 5,422,203; U.S. Pat. No. 5,626,985 and U.S. Pat. No. 5,630,993.
When the electrolytic component is in the form of a dry polymer matrix, it consists of a high molecular weight homo or copolymer, which is cross-linkable or non cross-linkable and includes a heteroatom in its repeating unit, such as oxygen or nitrogen for example, in which an alkali metal salt is dissolved such as LiN(SO2CF)2, LiSO3CF3 and LiClO4. Polyethylene oxide is a good example of a polymer matrix which is capable of solvating different alkali metal salts. Armand, in U.S. Pat. No. 4,303,748 describes families of polymers which may be used as electrolytic component ill lithium batteries. More elaborated families of polymers (cross-linkable or non cross-linkable co-polymers and terpolymers) are described in U.S. Pat. Nos. 4,578,326; 4,357,401; 4,579,793; No. 4,758,483 and in Canadian Patent No. 1,269,702. The use of a high molecular weight polymer enables to provide electrolytes in the form of thin films (of the order of 10 to 100 xcexcm) which have sufficiently good mechanical properties to be used entirely as separator between the anode and the cathode while ensuring ionic transport between the electrodes. In the composite, the solid electrolyte serves as binder for the materials of the electrode and ensures ionic transport through the composite. The use of a cross-linkable polymer enables to utilize a polymer of lower molecular weight, which facilitates the preparation of the separator as well as the composite and also enables to increase the mechanical properties of the separator and, by the same token, to increase its resistance against the growth of dendrites when using a metallic lithium anode. Contrary to a liquid electrolyte, a solid polymer electrolyte cannot escape nor be evaporated from the generator. Its disadvantage results from a lower ionic mobility obtained in these solid electrolytes which restricts their uses at temperatures between 60 and 100xc2x0 C.
The gel electrolytic component is itself generally constituted of a polymer matrix which is solvating or non-solvating for lithium salts, an aprotic solvent and an alkali metal salt being impregnated in the polymer matrix. The most common salts are LiPF6. LiBF4 and LiN(SO2CF)2 and the polar aprotic solvents may be selected from propylene carbonate, ethylene carbonate, butyrolactones and 1,3-dioxolane, to name only a few. The gels may be obtained from a high molecular weight homo or copolymer which is cross-linkable or non cross-linkable or from a cross-linkable homo or copolymer. In the latter case, the dimensional stability of the gel is ensured by cross-linking the polymer matrix. Polyethers including cross-linkable functions such as alkyls, acrylates or methacrylates are good examples of polymers which may be used in formulating a gel electrolyte, such as described in U.S. Pat. No. 4,830,939. This is explained by their capacity to solvate lithium salts and their compatibility with polar aprotic solvents as well as their low cost, and ease of handling and cross-lining. A gel electrolyte has the advantage of being handled as a solid and of not escaping or going out of the generator as is the case with liquid electrolyte generators. Ionic transport efficiency is associated with the proportion of aprotic solvent incorporated in the polymer matrix. Depending on the nature of the polymer matrix, the salt, the plasticizing agent and its proportion in the matrix, a gel may reach an ionic conductivity of the order of 10xe2x88x923 Scmxe2x88x921 at 25xc2x0 C. while remaining macroscopically solid. As in the case of a dry electrolyte, a gel electrolyte may be used as separator between the anode and cathode while ensuring ionic transport between the electrodes. In the composite electrode(s) of the generator, the gel electrolyte is used as binder for the materials of the electrode(s), and ensures ionic transport through the composite electrode(s). However, the loss of mechanical property resulting from the addition of the liquid phase (aprotic solvent) should generally be compensated by the addition of solid fillers, by cross-linking the polymer matrix whenever possible, or in some cases, when the proportion of liquid is too high, by using a porous mechanical separator which is impregnated with the gel which serves as electrolytic component in the separator. Examples of a generator utilizing a gel electrolytic component are described in U.S. Pat. No. 5,443,927 and U.S. Pat. No. 4,830,939. Takeda et al., in U.S. Pat. No. 5,658,687 claim a battery and a process of manufacturing said battery which comprises an electrolytic component consisting of a specific, cross-linkable and high molecular weight polyether. This high molecular weight polyether is obtained by esterification of a polyethylene oxide glycol in the presence of acrylic acid or methacrylic acid, sulfonic acid or para-toluenesulfonic acid and an organic solvent. The authors mention that the addition of an organic solvent such as a cyclic carbonic ester for example, which means the formation of a gell electrolyte, enables to substantially increase the conductivity of the electrolyte. The electrolytes thus obtained from said polyether can be used as separator, and in the composites, as electrolytic material and as binder for the material of the electrode. In composite electrodes, a second polymer may be added in small proportion to the polyether which is used as electrolyte. Generally, this second polymer is added for the purpose of substantially increasing the mechanical properties of the composite.
The poor resistance of polyethers towards oxidation is however an important problem which is associated with the utilization of solid and gel electrolytes based on polyether as the electrolytic material in a composite cathode in which the voltage in recharge may reach and even exceed 3.5 to 3.7 V. This results in an important loss of capacity of the generator which is caused by the more or less massive degradation of the polymer matrix during consecutive cycles of discharge/charge.
The present invention concerns a new concept of electrolytic component including more tan one polymer matrix which is swellable by a liquid electrolyte consisting of at least one aprotic solvent and at least one alkali metal salt, with different amounts of solvents, the matrices being macroscopically separated inside the generator. The rate of differential swelling enables to locally optimize the properties of conductivity such as in composite electrodes and the mechanical properties of the polymer membrane which acts as mechanical separator.
The present invention concerns an electrolytic component for an electrochemical generator, said electrolytic component consisting of at least two polymer matrices containing at least one alkali metal salt and at least one polar aprotic organic solvent. Said electrolytic component is defined as the electrolytic material which constitutes the separator and the electrolytic material of at least one composite electrode. The polymer matrices are macroscopically separated therein and one of them must be a polyether which is localized wholly or in part in the separator of the generator. The term macroscopic separation means that the polymer matrices are not interpenetrated in the form of microscopic mixtures, such as in an interpenetrated network. The polymer matrices are selected so as to locally optimize the amount of aprotic solvent, in order that the latter be unequally distributed between the polymer matrices. The term swellable polymer matrix means a polymer matrix which can include an amount of aprotic organic solvent which is sufficient to form a gel.
It is shown in the present invention that the use of a separator based on a gel polyether does not limit the power, i.e. the capacity of charge or discharge of the generator as compared to an equivalent generator in which the separator consists of a free liquid solvent included in the pores of an inert porous separator. This comparison obtained by utilizing a cross-linked separator in order to limit the amount of solvent in spite of the use of a polyether matrix which is very compatible with the solvent, enables to optimize the mechanical behaviour of the polymer matrix and to optimize its thinness. On the other hand, and this is one of the advantages of the invention, it is also shown that it is possible and desirable to control the amount of solvent of the polymer matrix or matrices in the composite electrodes preferably with a higher amount of solvent so as to increase the ease of diffusion of alkali metal salt ions, which is generally more limiting in composite electrodes because of the tortuous paths resulting from solid fillers.